Only 3 to 10% of gas from tight shale is recovered economically through natural depletion, demonstrating a significant potential for enhanced shale-gas recovery (ESGR). Experimental studies have demonstrated that shale kerogen/organic matter has higher affinity for carbon dioxide (CO₂) than methane (CH₄), which opens possibilities for carbon storage and new production strategies.

This paper presents a new multicomponent adsorption isotherm that is coupled with a flow model for the evaluation of injection/ production scenarios. The isotherm is dependent on the assumption that different gas species compete to adsorb on a limited specific surface area. Rather than assuming a capacity of a fixed number of sites or moles, this finite surface area is filled with species taking different amounts of space per mole. The final form is a generalized multicomponent Langmuir isotherm. Experimental adsorption data for CO₂ and CH₄ on Marcellus Shale are matched with the proposed isotherm using relevant fitting parameters. The isotherm is first applied in static examples to calculate gas-in-place reserves, recovery factors (RFs), and enhanced gas-recovery (EGR) potential according to the contributions from free-gas and adsorbed-gas components. The isotherm is further coupled with a dynamic flow model with application to CO₂/CH₄ substitution for CO₂-ESGR, assuming only a gas phase exists in the system. We study the feasibility and effectiveness of CO₂ injection in tight shale formations in an injection/production setting representative of laboratory and field implementation and compare that with regular pressure depletion.

The production scenario we consider is a 1D shale-core or matrix system first saturated with free and adsorbed CH₄ gas with only the left-side (well) boundary open. During primary depletion, gas is produced from the shale to the well by advection and desorption. This process tends to give low recovery and is entirely dependent on the well pressure. Stopping production and then injecting CO₂ into the shale leads to an increase in pressure, where CO₂ is preferentially adsorbed over CH₄. The injected CO₂ displaces but also mixes with the in-situ CH₄. Restarting production from the well then allows CH₄ gas to be produced in the gas mixture. Diffusion allows the CO₂ to travel farther into the matrix while keeping CH₄ accessible to the well. Surface substitution further reduces the CO₂ content and increases the CH₄ content in the gas mixture that is produced to the well. A result of the isotherm and its application of Marcellus experimental data is that adsorption of CO₂ with resulting desorption of CH₄ will lead to a reduction in total pressure if the CO₂ content in the gas composition is increased. That is in itself an important drive mechanism because the pressure gradient driving fluid flow is maintained (pressure buildup is avoided). This is because CO₂ takes approximately 24 times less space per mole than CH₄.

致密页岩气只有3％到10％是通过自然消耗经济地回收的，这显示出提高页岩气回收率（ESGR）的巨大潜力。实验研究表明，页岩干酪根/有机物对二氧化碳（CO ₂）的亲和力高于甲烷（CH ₄），这为碳储存和新的生产策略提供了可能性。

本文介绍了一种新的多组分吸附等温线，该等温线与流动模型相结合，用于评估注入/生产情景。等温线取决于以下假设：不同的气体种类竞争吸附在有限的比表面积上。而不是假定容量为固定数量的位点或摩尔，该有限的表面积填充了每摩尔具有不同空间量的物质。最终形式是广义的多组分Langmuir等温线。CO ₂和CH _4的实验吸附数据使用相关的拟合参数，将Marcellus页岩上的等温线与建议的等温线匹配。等温线首先在静态示例中应用，根据游离气体和吸附气体成分的贡献来计算就地储量，回收因子（RF）和增强的天然气回收（EGR）潜力。假设系统中仅存在气相，则等温线还与动态流模型结合，该模型应用于CO ₂ / CH ₄替代CO ₂ -ESGR。我们在代表实验室和现场实施的注入/生产环境中研究了致密页岩地层中CO ₂注入的可行性和有效性，并将其与常规压力消耗进行了比较。

我们考虑的生产方案是一维页岩岩心或基质系统，该系统首先被游离的和吸附的CH ₄气体饱和，仅左侧（井）边界开放。在一次采油过程中，通过对流和解吸，从页岩到井中产生了天然气。该过程往往使采收率低，并且完全取决于井压。停止生产，然后将CO ₂注入页岩会导致压力升高，其中CO ₂的吸附优先于CH _4的吸附。注入的CO ₂取代了原位CH ₄并与之混合。从井中重新开始生产，然后允许CH ₄在混合气体中产生的气体。扩散使CO ₂可以更深地进入基质，同时使CH ₄易于进入井中。表面置换进一步降低了生产到井中的气体混合物中的CO ₂含量并增加了CH ₄含量。等温线和其马塞勒斯实验数据的应用的结果是CO的吸附₂与CH的所得解吸₄将导致总压力的减小，如果CO ₂气体成分中的含量增加。这本身就是重要的驱动机构，因为保持了驱动流体流动的压力梯度（避免了压力升高）。这是因为，CO ₂每摩尔占据的空间比CH ₄少大约24倍。